1. Field of the Invention
The present invention relates to a process of producing cold-bonded iron ore to be used as a charge of a blast furnace and, more particulary, to a process of producing non-fired agglomerated iron ore to be used as the blast-furnace charge.
2. Description of the Prior Art
The properties required for the cold-bonded iron ore to be used in the blast furnace are itemized into the following four points: (1) it has a high cold strength; (2) it has a large angle of repose; (3) its disintegration during reduction is little; and (4) it has a high softening temperature. In the prior art, however, there is no cold-bonded iron ore which has succeeded in sufficiently satisfying those requirements.
As a representative of the cold-bonded iron ore, there are cold-bonded pellets which are prepared by adding a binder and a suitable amount of water to fine iron ore, as shown in the flow chart of FIG. 1. According to this prior art process, the iron ore is first ground. The ground iron ore fines are then mixed with additives and portland cement and are humidified. The resultant mixture is pelletized for subsequent classifications. The over-sized pellets are cured so that they may be hardened. The remaining undersized pellets are returned so that they may be reused in the process. The hardened pellets are classified again so that the over-sized may be used as a charge of the blast furnace when they attain a sufficient handling strength. On the other hand, the under-sized pellets are returned as one of the starting materials so that they may be subjected again to the iron-ore producing cycle thus far described. In this cycle, the ground iron ore fines usually have a particle diameter not exceeding 0.4 mm, and the cement acting as a hydraulic binder is not limited to the portland cement but can be blast-furnce cement or blast-furnace slag.
Nevertheless, the pellets thus produced have a defect that their distribution in the blast furnace is uneven because they have so spherical a shape that their angle of repose is small. More specifically, it is necessary for the stable operations of the blast furnace that the charge be distributed in a desired state, i.e., be overlaid evenly in the radial direction of the furnace. For that necessity, the charge such as the pellets is so adjusted that it is fed to the blast furnace with a radially even distribution. In case the charge is composed of spherical pellets, that adjustment is less effective so as to raise a problem when a plenty of the pellets are used, because they have a small angle of repose. In other words, the charge is concentrated locally at the center of the furnace to undesirably raise the ore/coke ratio at said position. As one answer for that problem, there has been made a proposal in which the pellets are crushed to endow a large angle of repose (as has been disclosed in Japanese Patent Laid-Open No. 56-35732).
It is further necessary to maintain the evenness of the gas flow through the blast furnace. Despite this fact, the gas flow loses its evenness because the cold-bonded pellets having the small angle of repose gather to the center portion of the blast furnace when they are fed thereto. In order to eliminate that defect intrinsic to the pellets, it is necessary to prepare a cold-bonded iron ore, which has a large angle of repose and its granular composition relatively resembling that of the sintered iron ore, thereby to make the blast-furnace charge distribution similar to that of the sintered iron ore so that the gas flow in the furnace may be made even to a satisfactory extent.
For this necessity, as is adopted in the sintered ore, there is a method in which the shapes of the cold-bonded pellets are improved by the crushing step. The pellets have a tendency to have an excessively small size for use as the charge of the blast furnace if they are crushed. Moreover, the pellets are liable to be disintegrated because their insides are loose despite their surfaces are sufficiently tight. Therefore, it has been believed that the improvement in the shapes by the crushing process is not preferable from the standpoint that the cold-bonded pellets are liable to be disintegrated.
In order to maintain the operation of the furnace under a good condition, the charge should have good properties at high temperatures (which will be shortly referred to as "hot properties"), particularly a high softening temperature. This softening temperature of the charge is dependent upon its chemical composition. More specifically, the gangue mineral contained in the charge reacts with iron monoxide (FeO) to produce a fluxing oxide having a low melting point, and this oxide melts to reduce the resistance to deformation thereby to deteriorate the hot properties under load and the air permeability, thus making the stable operation of the blast furnace difficult.
As methods of improving the hot properties of the cold-bonded pellets, therefore, there have been known to the prior art: (1) a method of improving the gangue mineral composition; and (2) a method of improving the reducibility of the pellets, as have been disclosed in Japan Patent Publication No. 53-13402 and in Japanese Patent Laid-Open Nos. 52-133003 and 52-117218, for example. However, those two methods (1) and (2) raise the following two problems [1] and [2]: